Diffusion transfer photographic elements are well known; they typically are in integral unit form and comprise a light-sensitive sheet comprising a support having thereon silver halide emulsion layers having associated therewith color image-providing compounds, a cover sheet comprising a support having thereon a neutralizing layer and a neutralization timing layer, and an alkaline processing composition; details of such photographic elements are given in, for example, Belgian Pat. Nos. 732,985 and 757,959, Japanese Patent Publication No. 33679/73, etc.
Other examples of such diffusion transfer color photographic elements use a light-sensitive sheet which has a layer containing an ultraviolet absorbing agent at a position remote the support thereof (topmost layer); such are described in, for example, Research Disclosure, No. 152, 15162 (November 1976), etc.
Also known in the art are cover sheets involving the use of a neutralizing layer comprising an acid polymer, e.g., a homopolymer of acrylic acid or copolymers of acrylic acid and acrylic acid esters and one or more neutralization timing layers containing, e.g., acetyl cellulose, styrene-maleic anhydride copolymers, a copolymer latex of vinylidene chloride, a methylacrylate and acrylic acid and a polymethylmethacrylate latex, which layers are provided in this sequence from the support. Various neutralizing layers are disclosed in U.S. Pat. Nos. 3,362,819, 3,765,885 and 3,819,371, French Pat. No. 2,290,699, etc., and various neutralization timing layers are disclosed in U.S. Pat. Nos. 3,455,686, 3,421,893, 3,785,815, 3,847,615 and 4,009,030, in Japanese Patent Application (OPI) No. 14415/77 (the term "OPI" as used herein refers to a "published unexamined Japanese patent application"), etc. In addition, latex based neutralization timing layers are described in Japanese Patent Applications (OPI) 70839/80, 138432/79, 138433/79 and 128335/79, in Research Disclosures, 18452 and 18564, etc.
Cover sheets in such elements serve various purposes. For example, such are not only used to provide a space over a light-sensitive sheet where an alkaline processing composition can be uniformly spread, but can also have the function of controlling pH, i.e., the pH of an alkaline processing composition, which is usually 13 or higher, can be reduced to 7 or below after a prescribed time has passed after spreading of the composition. To impart such a pH controlling function to a cover sheet, a neutralizing layer (which neutralizes alkali introduced from a processing composition) and a neutralizing timing layer (which controls the time for pH reduction) are provided in or on the cover sheet.
In diffusion transfer color photographic elements, a light-sensitive sheet and a cover sheet are superposed such that the coated layer of the light-sensitive sheet is in contact with the coated layer of the cover sheet, as disclosed in, e.g., Belgian Pat. No. 757,959 and Japanese Patent Applications (OPI) Nos. 153628/75 and 153629/75.
Such integrated photographic elements are, in general, used in pack form as described in Research Disclosure, No. 187, 18714 (Nov. 1979), and the resulting pack element is loaded in a camera. The pack has a spring on one side thereof, and, due to the force of the spring, the light-sensitive sheet and the cover sheet of the photographic element are kept in intimate contact, i.e., the topmost coated layer of the light-sensitive sheet and the topmost coated layer of the cover sheet are in firm, direct contact until a photograph has been taken, supports for each forming a "sandwich" therefor.
As earlier described, an ultraviolet absorbing agent-containing layer can be employed as the topmost layer of the light-sensitive sheet and a neutralization timing layer can be employed as the topmost layer of the cover sheet. However, when such sheets are stored for a long period of time in intimate contact, oil-soluble additives contained in the light-sensitive sheet, e.g., an ultraviolet absorbing agent, a dispersing oil, a surface active agent, etc., can migrate into the neutralization timing layer of the cover sheet, and this phenomenon can markedly occur, especially at high temperature.
On the other hand, when an alkaline processing composition is spread over the surface of a neutralization timing layer, the timing layer must form a controlled permeability barrier to alkali to maintain the processing composition in a state of high alkalinity for a definite period of time, and after a desired time has elapsed it allows alkali to pass therethrough to reach the neutralizing layer laid therebeneath. In the neutralizing layer, the alkali contacted therewith is neutralized by acid polymer therein. Upon neutralization of the alkali by the acid polymer, silver development, chemical reaction of dye couplers with the oxidation products of developing agents and dye diffusion are halted. Thus, the neutralization timing layer functions to control the time elapsed before neutralization begins.
However, migration of oil-soluble compounds (such as an ultraviolet absorbing agent, an oil used as a dispersing aid, etc.) into the neutralization timing layer frequently alters the permeability of the neutralization timing layer to alkali (usually such lowers the permeability). It is thought that such a phenomenon results from plasticization of the neutralization timing layer, and this phenomenon is markedly observed in the case where the neutralization timing layer is present at the direct interior of a sandwich as earlier described and contains a polymer latex.
Reduction in neutralization timing time due to lowering the alkali-permeability of the neutralization timing layer results in a premature cessation of various steps necessary to produce an image, e.g., silver development, chemical reaction of dye forming compounds with oxidation products of developing agents, dye diffusion, etc., before the image formation has adequately proceeded. Consequently, high image density cannot be attained. It has been desired in the art to remove this problem.
Some approaches have been suggested to render compounds such as oils, ultraviolet absorbants, etc., resistant to diffusion into the neutralization timing layer. For instance, per one approach the molecular weight of such a compound is increased while in another approach the binder content of an ultraviolet absorbing layer is increased in order to render such a compound therein diffusion resistant. However, an increase in molecular weight of such a compound is attended by disadvantages such as the synthesis of an oil or an ultraviolet absorbant becomes difficult and the thickness of a layer containing such a compound increases. On the other hand, an increase in binder content causes an increase in thickness of the ultraviolet absorbing layer, whereby completion of image formation is delayed.